ORIGINAL RESEARCH

 

Taxonomic significance of the abaxial lemma surface in southern African members of Helictotrichon (Poaceae)

 

 

Aluoneswi C. Mashau^{I, II}; Lyn Fish^I; Abraham E. van Wyk^{II, III}

^IBiosystematics & Research Collections Division, South African National Biodiversity Institute, South Africa
^IIDepartment of Plant Science, University of Pretoria, South Africa
^IIIH.G.W.J. Schweickerdt Herbarium, University of Pretoria, South Africa

Correspondence

 

 

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ABSTRACT

BACKGROUND: Helictotrichon (excluding Amphibromus, Avenula, Helictochloa and Tricholemma), a genus of temperate C₃ grasses, is represented by 14 species in southern Africa. Members of the genus are difficult to identify at species level on the basis of macromorphology alone
OBJECTIVES: The primary objective of this study was to investigate the usefulness of the micromorphology of the lemma surface for differentiating amongst the southern African members of Helictotrichon
METHOD: Lemma surfaces were studied with scanning electron microscopy (SEM). Lemmas were obtained from herbarium specimens housed in the National Herbarium, Pretoria (PRE). These were mounted on aluminium stubs using double-sided adhesive tape, sputter-coated with gold, and photographs were taken using a J840 scanning electron microscope
RESULTS: Based on whether the lemma surface is smooth, papillate, scaberulous, scabrid or a combination of these, five groups of species are distinguished. All species, except Helictotrichon barbatum which has a smooth lemma surface, have small prickles (scaberulous). Helictotrichon leoninum, Helictotrichon quinquesetum and Helictotrichon rogerellisii have only small prickles, whilst Helictotrichon dodii, Helictotrichon hirtulum, Helictotrichon namaquense and Helictotrichon roggeveldense have, in addition, medium to large prickles (scabrid). Helictotrichon capense, Helictotrichon longifolium, Helictotrichon longum and Helictotrichon turgidulum have a combination of small prickles and papillae whilst Helictotrichon galpinii and Helictotrichon natalense have a combination of all three. A key to the groups and photos of the different types of surfaces are provided
CONCLUSION: The micromorphology of the lemma surface was shown to be of considerable taxonomic significance and to be extremely useful for differentiating amongst species

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Introduction

Helictotrichon Besser ex Schult. & Schult.f. (Mashau, Fish & Van Wyk 2010b) [excluding Amphibromus Nees, Avenula (Dumort.) Dumort., Helictochloa Romero Zarco and Tricholemma (Röser) Röser] (Romero-Zarco 2011; Röser et al. 2009) is a genus of C₃ grasses belonging to the subfamily Pooideae and tribe Poeae (Soreng, Davis & Voionmaa 2007) in the Poaceae. The genus, with about 40 species worldwide (Gibbs Russell et al. 1990; Mabberley 2008), is most diverse in the temperate regions of the northern hemisphere, especially Europe, from where it extends southwards through the East African mountains (with about five species) to southern Africa, with 14 species in the Flora of southern Africa (FSA) region and one species in Madagascar. Biogeographically the link between the African and Mediterranean members of the group is via Frank White's Afromontane Archipelago-like Regional Centre of Endemism (Van Wyk & Smith 2001).

Schweickerdt (1937) revised Helictotrichon in South Africa and recognised 12 species, one of which was only known from the type collection, and a number of other species only from a very few herbarium specimens. The present availability of more herbarium collections has necessitated a revision of the group for the FSA region, this having already resulted in the description of two new species (Mashau, Fish & Van Wyk 2010a).

Southern African members of Helictotrichon have an inflorescence which is a narrow, open, contracted to spike-like panicle with solitary and pedicelled spikelets. Spikelets are 7 mm - 30 mm long (excluding awns), laterally compressed and disarticulate above the glumes and between the 2-6 florets. Glumes are usually shorter than the spikelet, equal or unequal, hyaline or subhyaline, and the apex is acute, acuminate or shortly awned. Lemmas are firmer than the glumes, being firmly membranous to leathery, and they have scarious or hyaline apices. The lemma is rounded dorsally, 5-11-nerved, the apex acute or acuminate and 2-lobed (rarely 4-lobed). The awn, which is longer than the body of the lemma, is geniculate and twisted, or only slightly so, and the callus is hairy. The palea is shorter than the lemma, 2-keeled and ciliate.

As many species of Helictotrichon are difficult to identify using only macromorphology, the primary objective of this study was to investigate the usefulness of the micromorphology of the lemma surface for differentiating amongst the southern African members of Helictotrichon. The elucidation of new features to facilitate identification will contribute towards knowledge of the southern African flora in general and infrageneric diversity of Helictotrichon in particular. Moreover, most species of Helictotrichon are relatively valuable grazing grasses as they remain green until late in winter. Reliable species identification will therefore benefit many end users of plant names, including ecologists, pasture scientists and farmers.

 

Research methods

Materials

The abaxial lemma surface features were studied in14 species of Helictotrichon (Table 1) from the FSA region. Lemmas were dissected from herbarium specimens housed in the National Herbarium (PRE), Pretoria (Table 1).

Procedures

Dry lemmas (24 samples) were mounted on aluminium stubs using double-sided adhesive tape, sputter-coated with gold and viewed under a Jeol J840 scanning electron microscope (SEM). The entire lemma, as well as enlargements of the middle-dorsal and distal (below the awn insertion) areas were photographed and compared. Terminology used to describe the surface features follows Ellis (1979).

 

Ethical considerations

The study did not involve the collection of any fresh samples as only previously collected dried herbarium material was used.

 

Results

The lemma provides some of the most useful diagnostic features to differentiate amongst the species of Helictotrichon in the FSA region. In some species the micromorphology of the abaxial lemma surface is variable from the base to the point of awn insertion, but the particular pattern of variability appears to be constant for a species. Taxonomically useful lemma characters as seen with the SEM are compared in Table 2. The main character states are whether the lemma surface is smooth, papillate, scaberulous (prickles small) or scabrid (prickles medium or large) (Figures 1a-h and 2a-f). Prickles are described as small if they are shorter than 42 μm, and as medium or large if they are longer than 56 μm. Based on lemma surface micromorphology the species can be classified into five groups. These are keyed out in Box 1.

 

 

Group 1: Lemma surface smooth

Helictotrichon barbatum. A smooth lemma is the most obvious character distinguishing H. barbatum from all other southern African members of Helictotrichon (Figure 1a).

Group 2: Lemma surface papillate and scaberulous

Helictotrichon capense, Helictotrichon longifolium, Helictotrichon longum and Helictotrichon turgidulum (Figure 1b, g & h; 2f).

Group 3: Lemma surface papillate, scaberulous and scabrid

Helictotrichon galpinii and Helictotrichon natalense are the only two species which have a combination of all the features that may be present on the lemma surface, namely papillae, small prickles and medium or large prickles (Figure 1d; 2b).

Group 4: Lemma surface scaberulous and scabrid

Helictotrichon dodii, Helictotrichon hirtulum, Helictotrichon namaquense and Helictotrichon roggeveldense (Figure 1c & e; 2a & e).

Group 5: Lemma surface scaberulous

Helictotrichon leoninum, Helictotrichon quinquesetum and Helictotrichon rogerellisii (Figure 1f; 2c & d).

 

Discussion

Although Schweickerdt (1937) included the lemma surface characters for each species description in his key, he only used it to separate some species that are closely related and then usually only as one of the many distinguishing characters used. In southern Africa, Chippindall (1955) closely followed Schweickerdt's key, whilst Stapf (1899) mentioned the lemma surface in the species descriptions but did not use it at all in the key. Descriptive terms such as granulate, finely or coarsely granular, scaberulous and scabrid are used by the various authors, but there is no doubt that they all refer to the prickles as seen under the SEM and referred to as such in the present contribution.

Helictotrichon dodii, with its very dense contracted panicle and long lemma lobes (6 mm - 8 mm, including awn), is a fairly well defined species. However, it does resemble H. turgidulum, which has shorter lemma lobes (3 mm - 5 mm, including awn); it also resembles H. natalense, which has a shorter spikelet (7 mm - 9 mm), and H. longifolium, which has setaceous leaves.

In addition to all these characters, the SEM shows that the lemma surface of H. dodii has small (scaberulous) as well as medium to large prickles (scabrid). This makes it easily distinguishable from H. turgidulum, H. natalense and H. longifolium, which have lemma surfaces with small prickles (scaberulous) mixed with papillae. Helictotrichon capense and H. longum, both winter rainfall species, fall in the same group as H. longifolium, a summer rainfall species from the mountains of Lesotho, KwaZulu-Natal and Eastern Cape, and H. turgidulum, the most widespread but predominately summer rainfall species.

Helictotrichon roggeveldense resembles H. namaquense in having a scabrid lemma and conspicuously hairy keels of the palea, but the two species differ in a number of other characters easily seen under the light microscope, such as inflorescence pulvinii and anther length (Mashau et al. 2010a). Helictotrichon roggeveldense occurs in the Sutherland District (Roggeveld Subcentre, Hantam-Roggeveld Centre of Endemism) (Van Wyk & Smith 2001) whilst H. namaquense is found mainly in the Kamiesberg, Namaqualand (Kamiesberg Centre of Endemism), with an outlier on the Hantamsberg, Calvinia. The SEM shows that the lemma surface of these two species has a combination of small (scaberulous) and medium to large prickles (scabrid).

Helictotrichon galpinii is easily distinguished by the broad glumes that are as long as the spikelet and the apparently scabrid lemma surface which is easily seen under a light microscope. However, the SEM images show the lemma surface of H. galpinii matching that of H. natalense as both have a combination of all three lemma surface types, namely papillate, small and medium to large prickles, present. Helictotrichon natalense occurs at lower altitudes (1900 m) than H. galpinii and both are summer rainfall species.

Helictotrichon rogerellisii is similar to H. longifolium in that both species have setaceous leaves and loosely flowered spikelets. However, they differ in their distribution (geographical range) as H. rogerellisii has only been found in the Bredasdorp District, Western Cape (Agulhas Plain Subcentre, Cape Floristic Region), in fynbos, mainly in shallow, humic soils between limestone outcrops on coastal plain. Helictotrichon longifolium is found along the Drakensberg Range, centred in Lesotho (Drakensberg Alpine Centre), especially in grasslands, and mainly on moist and rocky mountain slopes. These two species also differ in a number of other characters (Mashau et al. 2010a) and under the SEM the lemma surface of H. longifolium has papillae and small prickles (scaberulous) (Group 2) whereas H. rogerellisii has small prickles only (Group 5).

Using macromorphological characters, H. leoninum is the only southern African species with the rachilla internodes glabrous - this character state makes the species distinctly different from all others. The lemma surface of H. leoninum, H. quinquesetum and H. rogerellisii are covered only with small prickles (scaberulous; Group 5). All three species occur in the winter rainfall area in and around Cape Town, to as far east as Bredasdorp.

Although the lemma surface in the FSA species of Helictotrichon is not sufficiently variable to distinguish between all species, it is still a very useful character for identification. It also allows for species that are possibly closely related to be grouped together. The use of the SEM enhances this character especially to distinguish whether the lemma surface is smooth, papillate or has small prickles mixed with larger prickles - details not seen or not easily identified under the light microscope.

 

Acknowledgements

A special word of thanks to Chris van der Merwe and Alan Hall of the Laboratory for Microscopy and Microanalysis, University of Pretoria, for assistance in using the SEM. The South African National Biodiversity Institute is thanked for financial support. Two anonymous reviewers are thanked for their comments on the manuscript.

Competing interests

The authors declare that they have no financial or personal relationships which may have inappropriately influenced them in writing this article.

Authors' contributions

A.C.M. (South African National Biodiversity Institute) prepared the samples and analysed the data, A.E.v.W. (University of Pretoria) was project leader and supervisor, and L.F. (South African National Biodiversity Institute) acted as mentor and made conceptual contributions.

 

References

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Soreng, R.J., Davis, J.I. & Voionmaa, M.A., 2007, 'A phylogenetic analysis of Poaceae tribe Poeae sensu lato based on morphological characters and sequence data from three plastid-encoded genes: Evidence for reticulation, and a new classification for the tribe', Kew Bulletin 62, 425-454.         [ Links ]

Stapf, O., 1899, 'Gramineae', Flora capensis 7, 472-477.         [ Links ]

Van Wyk, A.E. & Smith, G.F., 2001, Regions of floristic endemism in southern Africa: A review with emphasis on succulents, Umdaus Press, Pretoria.         [ Links ]

 

 

Correspondence:
Aluoneswi Mashau
Private Bag X101
Pretoria, 0001
South Africa
Email: c.mashau@sanbi.org.za

Received: 13 May 2013
Accepted: 26 Feb. 2014
Published: 25 Mar. 2015